Conventionally, for example, as described in JP-A-2003-318412, a semiconductor device having a diode for flowing current between an anode layer and a cathode layer is disclosed. The anode layer, a drift layer having an impurity concentration lower than the anode layer, and the cathode layer having an impurity concentration higher than the drift layer are stacked in this order.
Here, one of general performance required to the diode is low power loss. The power loss of the diode is shown as a sum of a stationary loss and a switching loss. The stationary loss is generated when forward current flows, and the switching loss is generated when a reverse current flows. The stationary loss has characteristics such that the stationary loss is reduced when an injection amount of a minority carrier to the drift layer becomes large. The switching loss has characteristics such that the switching loss is reduced when the injection amount of a minority carrier to the drift layer becomes small. Accordingly, a relationship between the stationary loss and the switching loss is a trade off relationship. On the other hand, in a conventional technique, the injection amount of the minority carrier is adjusted by controlling an impurity concentration distribution of the anode layer and the drift layer and/or by patterning the anode layer so that the stationary loss and the switching loss are adjusted.
However, since each of the above adjusting methods is performed during a manufacturing process of the diode (i.e., the semiconductor device), the injection amount and the accumulation amount of the minority carrier cannot be adjusted flexibly after the semiconductor device is manufactured. Thus, a problem occurs such that the stationary loss and the switching loss are not controlled. The stationary loss and the switching loss of the diode are varied according to usage environment of the semiconductor device. Thus, it is required to control them according to the usage environment.
Here, another method for controlling the stationary loss and the switching loss is a method for irradiating an electron beam on the drift layer so that a lifetime of the minority carrier is adjusted. However, this adjusting method is also performed during the manufacturing process of the semiconductor device. Thus, after the semiconductor device is manufactured, the stationary loss and the switching loss are not adjusted flexibly.
Further, a loss of a high breakdown voltage insulated gate type semiconductor device such as a trench gate type IGBT includes the stationary loss and the switching loss. These loss characteristics depend on the injection amount of the minority carrier from a collector.
FIG. 30 shows a cross sectional view of a conventional N channel type IGBT. As shown in this drawing, a N− type drift layer 302 is formed on a surface of a P+ type substrate 301 via a field stop layer (i.e., FS layer) 302a. The substrate 301 provides a collector region, and the FS layer 302a functions as a buffer layer. A trench gate structure is formed in a surface portion of the N− type drift layer 302. Specifically, a P type base region 303 is formed in the surface portion of the N− type drift layer 302. Further, a trench 304 is formed to penetrate the p type base region 303. The P type base region 303 is divided into multiple portions by the trench 304. A N+ type emitter region 305 is formed in a part of the portions of the region 303 so that a channel P layer 303a is formed. The N+ type emitter region 305 is not formed in the other part of the portions of the region 303 so that a float layer 303b is formed. Further, a gate electrode 307 is formed in the trench 304 via a gate insulation film 306. The gate electrode 307 contacting the channel P layer 303a provides the gate electrode 307a for applying a gate voltage. The gate electrode 307 not contacting the channel P layer 303a provides a dummy gate electrode 307b for a dummy electrode.
In the above IGBT, when an injection amount of a hole from the P+ type substrate 301 as the collector region is large in the on state, many holes are accumulated by using the FS layer 302a. Thus, a conductivity change is promoted largely. Thus, the stationary loss is reduced. On the other hand, in the IGBT, when the accumulation amount of the hole is large in the on state, a time period to remove the holes in case of turning off becomes large. Thus, a turn off loss increases.
Accordingly, it is required to control and design a balance between the stationary loss and the switching loss according to a driving frequency for usage so that a total loss of the stationary loss and the switching loss is minimized.
Accordingly, conventionally, in the life time control technique with using the electron beam irradiation, as shown in FIG. 30, a FS type IGBT is proposed such that the P+ type substrate 301 providing the collector region is polished to be thin, and the N+ type FS layer 302a is formed between the P+ type substrate 301 and the N− type drift layer 302. This is disclosed in, for example, JP-2003-101020.
In the lifetime control technique, the electron beam or the like is irradiated on the device, and then, the device is annealed in the manufacturing process of the device, so that a recombination center is generated in the drift layer. Thus, the lifetime of the minority carrier is adjusted. Accordingly, transport efficiency of the minority carrier is adjusted, and the design of the loss is optimized. In the FS type IGBT, a difference between a concentration in the P+ type substrate 301 for providing the collector region on the backside and a concentration in the N+ type FS layer 302a is controlled in the manufacturing process of the device so that the injection amount of the hole (i.e., the minority carrier) is adjusted. Thus, the design of the loss is optimized. With using these techniques, the injection of the minority carrier is optimized or the transport efficiency is adjusted according to an application of the device.
However, the above techniques are applied to customize the device in the manufacturing process of the device. Thus, the techniques lack general versatility of the device. Further, even in one application of the device, the environmental conditions such as temperature and operating conditions such as a driving frequency are varied. Thus, the techniques cannot match the change of conditions.